The Forerunners' Halo Array is a network of immense rings. Its purposes include securing Forerunner pre-eminence in this account. Firing it would kill sentient beings throughout the galaxy, denying the Flood hosts and eventually starving it. Destroying the populations the system is supposed to defend makes the meaning of a successful defense difficult to resolve.
Chapter · 2:00:02-2:01:42
The Halo Array denies the Flood its hosts
Firing the rings would kill sentient life across the galaxy to starve the Flood.